Why is it advantageous for blood pressure to be high in arteries and low in veins?

I'm confused. Surely increasing the blood pressure in veins would help movement, and obviate the need for valves?

I know that the heart's pulses are felt far more strongly in the arteries, so presumably higher pressure there enables these pulses to move the blood along them. Is this true?

Thanks in advance if you can help,

Patrick.

Answers below supplied by two of my colleagues a cardiac physiologist:

answer 1
Why is it advantageous for blood pressure to be high in arteries and low in veins?

The cardiovascular system consists of two circuits, systemic, from left ventricle around the various organs (largely arranged in parallel) back to the right atrium and the pulmonary circulation from the right ventricle through the lungs to the left atrium.  The circulation of blood around the both these circuits requires a pressure gradient from arteries at high pressure to veins at low pressure.

Surely increasing the blood pressure in veins would help movement, and obviate the need for valves?

No.  Because the pressure gradient is reduced, there would be a problem with  adequate circulation.  In addition, the capillaries, where the business of gas and nutrient exchange takes place, have permeable walls. An increase in venous pressure would increase the pressure inside the capillaries leading to 'leak' of fluid from the blood into the tissue (a bit like tea being squeezed from a tea bag).  The tissue becomes 'water-logged' (clinicians and scientists call this 'oedema') and it is a problem because it increases the distances that gases and nutrients must diffuse to meet the metabolic needs of the tissue. This problem happens in congestive heart failure, where the venous pressure increases due to inadequate pumping by the heart.


I know that the heart's pulses are felt far more strongly in the arteries, so presumably higher pressure there enables these pulses to move the blood along them. Is this true?

Pretty much.  Of course, the 'pulse' is a pressure wave caused by the oscillation in pressure due to the pumping action of the ventricles. This pressure wave travels a lot further at each beat than the actual
movement of blood.  The key point is that it is the pressure gradient around the circulation that keeps the blood moving.

answer 2
The questionner is right to resume that pressure is needed to cause blood to flow - I quote Ohms Law to students as the factors that causes an electric current to flow apply in broad terms to fluid.

Evolution could I suppose have given us a system that used the heart to generate sufficient pressure to get blood right round the circulation.  But there are very good reasons for it not having happened:

1.     The pressure required would be very much greater than  it is, especially to overcome the effects of gravity on the way back from the legs.  That would mean a much larger heart and much stronger arteries, with greater risks than we already have from the significant pressures in arteries.

2.    The essence of the cardiovascular system is to supply nutrients and remove waste from the tissues.  That happens in the capillaries and relies absolutely on the capillary walls being so thin that compounds can get across in an appropriate manner; it's an extremely delicate balance.  That means the walls are weak and so cannot support a large pressure within.

3.    Gravity is a huge complication.  It helps blood back to the heart form the head but holds it back in the lower body - the more so the larger the animal.  A centrally-controlled pressure system would have difficulty adjusting to the very different effects of gravity in different parts of the body.

So evolution has found a different manner of ensuring that blood circulates; enough pressure from the heart to get in a controlled way to the tissues around the body, but different mechanisms for getting it back to the heart.  Those mechanisms are in turn clever.  When we exercise we use more oxygen and nutrients, and to support that we require more blood to circulate.  The activity of exercise in the muscles compresses the thin-walled veins and helps push the blood back; and our vigorous breathing not only sucks air into the lungs but (because the chest contains the heart as well as the lungs) helps to suck more blood back to the heart.  The valves not only ensure one-way flow in the veins, but also break down the column of blood to lessen the effects of gravity; witness the imbalance at the capillaries and resultant swelling of the feet when the valves cease to function in the condition known as 'varicose veins'.